When did our ancestors first start to use tools? That is a very interesting question, and immediately leads to the question of how we could know. What kind of evidence could there be to establish human tool use?

The most obvious evidence would be the tools themselves. Fortunately stone tools are made of stone, which is highly durable and can last millions of years. Earlier this year researchers published evidence of the now earliest known stone tools, from 3.3 million years ago. This finding was significant for several reasons.

First, it pushed back tool use 700,000 years. Perhaps even more significant, prior to this find the oldest confirmed stone tools were associated with the species Homo habilis, the first of the Homo genus that led to modern humans. It could therefore be said that sophisticated tool use (meaning modified tools that can be recognized) was unique to the Homo genus. These new tools, called the Lomekwian tool culture, predate Homo habilis, and are likely associated with Australopithecus afarensis.

Finally, these tools fill in the gap between the sophisticated tool making of Homo habilis and the primitive tool using techniques observed in modern day chimps. Homo habilis, using what we call the Oldowan tool culture, would precisely flake off pieces of stone from a core in order to make sharp flakes. Chimps, however, use unmodified stone in a two-handed overhead bashing technique. The Lomekwian tools are larger that Oldowan tools and are more primitive (but still clearly modified), and likely represent more of a two-handed bashing technique.

All of this raises an interesting point – seeing that a stone has been modified into a tool is one window into hominin tool use. However, it seems likely, even unavoidable, that earlier hominins used unmodified stones and sticks as tools. How would we know? Well, not simply by finding their tools.

This point applies to all of science; we can only peer through windows provided by available evidence, and are limited by the nature of that evidence. Complex multicellular life appears “suddenly” in the fossil record, when hard parts that fossilize evolved. But clearly there was multicellular life without hard parts previously. (We do find them also, but only scant evidence and probably not the actual ancestors to the Cambrian fauna.)

Getting back to tool use, there is another window of evidence – evidence of the results of tool use. Specifically, if a hominin used a stone as a tool to scrape meat off the bone of an animal, that would leave marks in the bone that could fossilize and survive.

This brings us to another study just published in the Journal of Human Evolution. Jessica Thompson et. al. have perhaps resolved a controversy regarding fossils found at the Hadar Formation at Dikika, Ethiopia. These finds date from about 3.4 million years ago, and again are likely associated in time and place with Australopithecus afarensis.

There are two bones with marks on them that were identified in a blind test by experts in such bone marks. Marks can be made by the environment, such as from trampling, or by tooth marks or stone cutting marks. Trampling marks tend to be shallow and curvy. Tooth marks have a “U” shaped groove, and stone cutting marks make a deep, straight “V” shaped groove. The experts identified the Dikika bones as being from stone tools.

This created a controversy, as it would push back the date and species of first verified tool use. Other scientists claimed the marks were trampling marks. The new study seeks to resolve the controversy with more evidence (go science).

Thompson and her team did an extensive survey of 4000 other bones from the same deposit. Many of these bones have trampling marks on them. They used statistical analysis to compare those bone marks with experimental trampling marks and with the two bones in question.

They found that the two bones were definite statistical outliers, meaning that they did not match the other bones from the site or the experimental trampling marks.

This does not prove that the marks on those two bone are from deliberate stone tool use, but it does show:

“The data show that the DIK-55 marks are outliers amongst bone surface damage in the Dikika area, and that trampling is not the most parsimonious interpretation of their origin.”

This leaves stone tool use as a viable explanation. Of course, reality is complex and quirky and it’s possible that some other unknown mechanism created the marks. When trying to reconstruct the past the best we can do is make the most logical inference from the data.

Conclusion

Several independent lines of evidence are converging on the conclusion that our hominin ancestors used tools. There is undeniable evidence of stone tool culture going back 2.6 million years, with smoking-gun evidence of the manufacture of deliberate stone tools and use of those tools to butcher animals.

The further back we go, however, the more uncertain the evidence becomes. It seems very likely that our ancestors did not immediately start making sophisticated stone tools (unless aliens came down and showed them how to do it, or they had their own Thomas Edison who developed it in one generation), and therefore it is likely that there was a long period of more primitive tool use, with more and more subtle evidence. This would lead all the way back to using completely unmodified tools to simply bash things, like modern chimps. (Chimps do modify sticks for use, but these would not survive in the fossil record.)

There may be an ultimate limit, therefore, to the kinds of evidence we can find to document the earliest tool use among our ancestors. We will probably push back the date of the earliest tool use even beyond where it is now, at 3.4 million year ago. But there will likely be a limit to how far back the evidence can go.